Field
The present invention relates to the manufacture of particles with controlled dimensions, particularly those having at least one embossed surface; a composition comprising such particles; and the use of such particles in inter alia paints, printing inks, spray paints, cosmetic products, high surface area ceramics or ceramic bodies, coloured plastics and optical elements or as an anti-counterfeit or security pigment.
Description of the Related Art
The preparation of embossed surfaces is well-known in the art, including the preparation of surfaces with a diffraction pattern or grating. A diffraction grating is formed when closely and regularly spaced grooves (typically 5,000 to 15,000 grooves per cm) of specified depths are embossed on a reflective surface. The diffraction grating produces an iridescent visual effect by diffracting incident light into its colour components by reflection from the grating. Thus, the viewer of the surface perceives a different colour depending on the orientation of the diffractive surface. Diffractive surfaces typically show the greatest effect under direct illumination from a controlled light source, rather than under diffuse light.
Diffraction patterns have been proposed for a variety of practical applications due to their aesthetic and utilitarian visual effects. Diffraction grating technology has been employed in the formation of two-dimensional holographic images which create the illusion of a three dimensional image to an observer, and these holographic images can form attractive displays. Information can be incorporated into and contained in the surface relief pattern of the hologram, specifically in the shape, depth, pattern and spatial frequency of the relief pattern. The images or information recorded on a hologram can be reconstructed by illuminating the surface relief pattern with light, which is diffracted and refracted by the surface relief pattern. The holographic information can be revealed under optical magnification only, if desired, or under illumination by light outside the visible spectrum. Thus, not only can diffraction and holographic patterns be used decoratively to create attractive visual effects, they can also carry images of varying size, as well as machine-readable information that function in the manner of a barcode. The use of holographic images has therefore been utilised in anti-counterfeiting applications and in security applications generally.
For the purpose of this application, the term diffraction grating includes holographic images that are based on diffraction grating technology.
The original diffraction gratings were formed by etching or scribing closely and uniformly spaced lines on polished metal surfaces. Subsequently, techniques were developed to reproduce a master diffraction grating by shaping a mouldable material against the master diffraction grating surface. More recently, thermoplastic films have been embossed by softening the surface of the films by elevated temperature and/or pressure, and passing them under stamping dies or embossing rollers in order to impart the diffraction grating or holographic image onto the softened surface. Alternatively, casting resin is coated onto a substrate web or an embossing drum prior to their surfaces coming into contact, and the resin then hardened by curing.
The original master (or parent) hologram is usually made on a photosensitive material, such as a photoresist film. The master hologram is physically delicate and is not used directly to mass-reproduce copies. Instead, a thin metal film (typically nickel) is electrodeposited on a master optical hologram in a manner which allows the surface relief pattern to be faithfully followed. Once the optical hologram and metal master are separated, a number of sub-masters are made from it. These sub-masters are then joined together in order to form a large sheet that can be wrapped around an embossing drum. The sub-master is typically referred to as the “shim”, although the term “shim” is also used to refer generally to the sheet containing a plurality of sub-masters, or the embossing drum on the surface of which is disposed the plurality of sub-masters. By using a correspondingly wide drum and substrate web, many replicas of the diffraction grating or hologram can be formed across the width of the web as well as along its length. A large number of replicas of the single hologram are produced by a single revolution of the drum.
A number of patents and patent applications have described the reproduction of optically variable effect structures such as holograms and diffraction grating using in situ polymerisation replication (ISPR) techniques in which a polymer is cast or moulded against a master diffraction grating surface, typically while the polymer is held on a substrate. The embossed polymeric material may be cured, if necessary, for instance by heat and/or radiation, and the structure of the diffraction grating is retained in the material. A curing step can take place whilst the polymeric material remains in contact with the master diffraction grating, or subsequently upon removal from the mould. Examples of this approach are described in U.S. Pat. Nos. 3,689,346, 4,758,296, 4,840,757, 4,933,120, 5,003,915, 5,085,514, EP-0540455-A, EP-0407615-A and GB-2027441. WO-99/38704-A (De La Rue) describes the preparation of security devices comprising a diffraction grating structure imparted on or into a radiation-curable coating on a shrinkable substrate. WO-94/18609 (Matthiesen) describes a process in which the diffraction grating structure is imparted on or into a radiation-curable coating layer disposed on a substrate, wherein the coating is cured whilst in contact with a master mould and before removal therefrom, and wherein the master mould is transparent to the curing radiation and is located between the curable coating and the curing radiation source.
The embossed surface of certain polymeric materials is sometimes sufficiently reflective that the optical effect of the diffraction grating occurs without further processing. More typically, however, it is necessary to metallize the polymer surface.
In some applications, the useful product is the embossed polymeric material itself. The embossed polymeric film can be cut into sections, each section carrying one or more repeat pattern(s) of the holographic image(s). Individual holograms may be separated from the web supporting material, if desired. The holograms may be applied to individual object surfaces by a hot-stamping process or by cutting the individual holograms from the web and adhering them to the object surfaces. In addition to the patents referenced above, examples of such disclosures include GB-2221870-A (De La Rue) which discloses a security device comprising a substrate on or in which is provided a diffraction grating which causes incident radiation to scatter in a random manner such that the scattered beams interfere to generate a complex speckle pattern. The security device is then verified by comparing the speckle pattern with a reference. The security device may comprise, or form part of, items such as labels, identification cards, bank cards, bank notes, stamps, travel or lottery tickets or security documents. The random scattering of incident light and complex nature of the speckle pattern is reported to reduce the likelihood of forgery via reverse engineering of the security device, and is stated as advantageous over prior security device systems such as those disclosed in U.S. Pat. No. 4,537,504.
In other applications, the embossed polymeric film is itself only an intermediate in the process and acts as a medium for the transfer of the diffraction pattern to other materials. For instance, in the preparation of metallic pigments, the embossed polymeric material acts as a template onto which a metallised layer is deposited. The metallised layer is subsequently removed and comminuted into individual metallic particles each of which carry at least a part of the repeated diffraction pattern. The embossed diffractive particles can be interspersed into liquid media such as paints or inks to produce diffractive compositions for subsequent application to a variety of objects.
U.S. Pat. No. 4,321,087 (Revlon Inc.) describes a process for the production of (un-embossed) thin metallic flakes comprising the steps of depositing a metal film onto a carrier sheet coated with a release layer, removing the metal from the carrier sheet and comminuting into metallic particles. The disclosure of U.S. Pat. No. 4,321,087 was utilised in conjunction with the technology of diffraction gratings and holograms in WO-93/23481-A (Avery Dennison Corporation) which discloses a process for making thin embossed metallic flake pigment, and coating and printing formulations containing them. The embossed pattern can be a machine-readable image such as a conventional bar code image or a holographic bar code image, and the embossed flakes are useful for security applications. In the process of WO-93/23481-A, a release coating is applied to a surface of a carrier sheet, and the outer surface of the release coating is embossed or provided with a diffraction pattern. Metal vapour is condensed in the form of a thin film onto the embossed outer surface of the release coating. The carrier sheet, having the release coating and the thin metal film thereon, is then passed through a solvent system which dissolves the release coating or the carrier; allowing most of the metal film to float off the carrier sheet into the solvent without destroying the embossment on the metal film. Any residual metal film can be recovered from the carrier sheet into a non-reactive liquid medium where it is dispersed into finer pigment particles by stirring or ultrasound techniques. The metallic pigment flakes may then be concentrated and formulated as desired.
WO-03/011980 also discloses the preparation of diffractive flake pigments having a diffractive structure formed on a surface thereof, the flakes having single or multiple layers. The multiple layer flakes may comprise a symmetrical stacked coating structure on opposing sides of a reflective core layer. U.S. Pat. No. 6,242,510 discloses the production of relatively large diffractive flakes particles. U.S. Pat. No. 5,912,767 discloses particles for use in a colour-shifting security ink in which the diffractive features are arranged on the particles in a circular arrangement having a specified groove frequency in order to obtain a uniform appearance. U.S. Pat. No. 6,112,388 teaches metallic particles containing an inorganic dielectric layer. Other variations on the technology are disclosed in U.S. Pat. Nos. 6,168,100, 5,549,774, 5,629,068 and 5,672,410.
WO-2005/017048-A and EP-1741757-A disclose the preparation of pigment flakes having a selected shape and/or symbol (such as a diffraction grating pattern) embossed on a surface thereof, to provide a covert security feature to an object. The selected shape or symbol is intended to be detectable under magnification using an optical microscope. The flakes are manufactured by depositing inorganic material onto a carrier sheet at least a portion of which comprises a pattern of embossed frames optionally within which are embossed the desired symbol(s). The deposited film is then separated from the patterned sheet and processed into flake. In this prior art, the embossed portion of the carrier sheet has a plurality of frames over which the inorganic material is deposited, and from which the deposited material must be stripped and processed into flakes, which are then incorporated into a security pigment composition. However, there remains a problem with this prior art in that the stresses associated with the deposition and stripping process result in stress-cracking and random fracture of the deposited material, resulting in incomplete or inadequate production of thin flakes which replicate the desired shape of the frame embossed in the carrier sheet. The prior art process utilises a relatively thick deposited inorganic layer, and therefore relatively thick flakes, in order for the desired shape of the embossed frame to be replicated in single flakes. Moreover, a pigment flake produced by the prior art process retains around its periphery the impression of the frame-lines present in the embossed carrier sheet. These peripheral frame impressions give rise to specious reflection and/or diffraction of light from the flake surface and reduce the efficiency of the reflective surface, and therefore the flake exhibits reduced brightness in the pigment composition. In addition, such flakes do not lie flat and may impair the flow characteristics of the vehicle in which they are incorporated. Similar processes are disclosed in EP-0978373-A.